Shows how to use the 
Combinable interface and the 
ShapeCombiner class to compute the intersection of a World Wind surface shapes with
 Earth's land and water.
 
 This example provides an editable surface circle indicating a region to clip against either land or water. The land
 and water are represented by an ESRI shapefile containing polygons of Earth's continents, including major islands.
 Clipping against land is accomplished by computing the intersection of the surface circle and the shapefile polygons.
 Clipping against water is accomplished by subtracting the shapefile polygons from the surface circle. The user
 specifies the location of the surface circle, whether to clip against land or water, and the desired resolution of
 the resultant shape, in kilometers.